This invention relates to the art of web forming wherein the web is formed from a dilute, aqueous slurry of fibers. More specifically, this invention relates to the formation of a paper web and, still more specifically, to the formation of a paper web on a so-called fourdrinier type of papermaking machine.
High speed papermaking machines can be broadly classified into two major types: fourdrinier and twin-wire machines. In twin-wire papermaking machines, a dilute, aqueous slurry of wood fibers is projected between a pair of converging forming wires which are travelling in the direction of the projecting stream. This aqueous stream of wood fibers is commonly referred to as "stock" in the paper industry. In twin-wire forming apparatus, stock dewatering is performed through both of the travelling forming wires so the stock jet stream is directed to the center of a throat defined by the converging forming wires to facilitate and promote equal drainage through each of the forming wires. This centered positioning of the stock jet also takes advantage of the fact that twin-wire formers usually operate with their co-running forming wires travelling in a non-horizontal path in order to utilize the force of gravity in urging water through each of the forming wires.
On the other hand, fourdrinier type papermaking machines are horizontally, or nearly horizontally, disposed since the water is only removed downwardly through a single forming wire on which the aqueous slurry of wood pulp fibers is deposited.
Accordingly, the stock jet cannot be directed into a non-existent throat between converging forming wires but must instead be directed onto the horizontally-arrayed fourdrinier forming wire. The web forming zone extends from the breast roll at the beginning of the fourdrinier to the couch roll at its end.
Over the years, as papermaking machine speeds have increased, different pulping methods were developed to more uniformly refine the wood pulp. Dewatering apparatus disposed beneath the fourdrinier wire were also developed and improved so quality paper could be formed at progressively higher speeds. However, the problem of spouting, wherein droplets of stock are projected upwardly from the fourdrinier wire by surface disturbances remained and increased in intensity as forming speeds increased. At lower papermaking speeds, and sometimes in consideration of other factors such as stock grades, spouting was not considered a problem, much less a major problem. Indeed, there was a time when so-called "shake" mechanisms were built into the fourdrinier section in order to produce or maintain a certain amount of stock agitation while it was in a fluid form on the fourdrinier wire before a sufficient amount of water was drained to form a cohesive web. It was thought, and still is, that a small, limited amount of stock agitation was beneficial to web formation.
However, at today's papermaking speeds, which range from about 2,000 feet per minute to about 5,000 feet per minute, even the extent of spouting activity on the fourdrinier wire which might have been considered tolerable, or even beneficial, at lower machine speeds, or in comparison with paper formed under lower standards in the past, have now become undesirable and unacceptable. In all cases, at high machine speeds, the degree of spouting must be controlled as excessive spouting is detrimental to the production of quality paper.
Prior attempts to produce better paper formation at increased speed included slanting the fourdrinier wire downwardly, or upwardly, or projecting the stock jet onto the fourdrinier wire as it travelled over the breast roll. However, each of these attempts had its own deficiencies and limitations to the net effect that the paper formed by such apparatus either was of inferior quality, or had to be produced at lower speed, or some combination of both.
Fourdrinier papermaking machines utilize a forming board which is a dewatering device located immediately downstream of the breast roll. A prime source of the spouting phenomenon on prior fourdrinier papermaking machines was the turbulence caused by impingement of the stock stream against the forming wire over the forming board. The forming board could not be removed since it was needed to both support the forming wire under the weight of the aqueous slurry of stock and control the drainage of the water during the initial, critical stage of paper formation.